The Nano is the smallest Arduino board available. The micro is a tad larger.

The Adafruit Trinket M0 is 27x15.3mm, about half the size of the Nano. But it does have I/O.

 

I don’t want to offend anyone. You all have provided important data.

I’m seriously looking at the ATTINY85. I’ve ordered a few.

... the nerve! ...

just kidding ...

I'm glad you've made up your mind and hope your choice serves you right. I'm not familiar with the attiny family, but I'll stay tuned to this thread, see how many new things I can learn.

Good luck!

 

I’m seriously looking at the ATTINY85. I’ve ordered a few.

The ATTiny85 is a nice little device. (I only used the ATTiny45, since the 85 hadn't come out yet -- !RSN! for 2 years or so.)

The neatest thing was the high-speed timer.

 

If you have selected ATtiny85 that is ok.
I can help you get started with your first "blinky" program using Atmel Studio and AVR Pocket Programmer.

 

I’ve just bought one chip and a programmer to try. Still open to other solutions.

 

You have to meet the following specs for I/O current (from ATTINY85 datasheet) -

4. Although each I/O port can sink more than the test conditions (10 mA at VCC = 5V, 5 mA at VCC = 3V) under steady state
conditions (non-transient), the following must be observed:
1] The sum of all IOL, for all ports, should not exceed 60 mA.
If IOL exceeds the test condition, VOL may exceed the related specification. Pins are not guaranteed to sink current greater
than the listed test condition.
5. Although each I/O port can source more than the test conditions (10 mA at VCC = 5V, 5 mA at VCC = 3V) under steady state
conditions (non-transient), the following must be observed:
1] The sum of all IOH, for all ports, should not exceed 60 mA.
If IOH exceeds the test condition, VOH may exceed the related specification. Pins are not guaranteed to source current
greater than the listed test condition.

The PSOC has a limitation per pin of 25 mA, but for whole chip 200 mA.


Regards, Dana.